Fix abs with char and short cuda types.

[gchanan]

Fixes #746.

I'm not really sure how the current code doesn't infinite loop (compiler optimizes it away? It infinite loops for me if I try to printf in there).

There are also two abs functions defined: see: http://stackoverflow.com/questions/8226705/g-abs-on-a-short-int-appears-to-turn-it-into-a-double.

We want the int one; the template version doesn't compile since you get errors like: "calling a host function("std::abs ") from a global function("kernelPointwiseApply2< ::Tensor_abs_Short_Op, short, short, unsigned int, (int)-1, (int)2> ") is not allowed"

[soumith]

ha! good catch.